Light-emitting diodes (LED) and LASER represent valid tools to provide photoinactivation in vitro and in vivo [
1‐
4]. 880 nm LED light provided a significant reduction of planktonic and biofilm of
Enterococcus faecalis and
Pseudomonas aeruginosa [
1‐
4]. However, the efficacy of the photoinactivation was dependent on the presence of endogenous photosensitizers, such as protoporphyrin IX (PpIX), inside the bacteria [
1‐
3]. Indeed, the photodynamic therapy (PDT) mechanism implies that the used photosensitizer molecule (PS) must be photoactivated by a light at specific wavelengths to induce its excited state that led to generate the reactive oxygen species (ROS) [
5]. An important aspect is that the photosensitizer such as PpIX is selectively accumulated more in abnormal or infected cells, without causing any damages to the healthy tissues [
6,
7]. Egli RJ and co-workers evidenced how PpIX differently accumulated in different cell types [
8]. For its characteristics, PDT can be a potential strategy in dental field, considering that periodontitis and peri-implantitis are bacterial inflammatory processes that promote the progressive bone loss, a condition difficult to solve [
9,
10]. Data from randomized clinical trials indicated that antibacterial PDT (aPDT) performed with a diode laser at 660 nm in combination with photosynthesizers such as phenothiazine chloride, methylene blue, or toluidine blue supported non-surgical periodontal treatments leading to significant improvements in all the investigated clinical parameters (PPD, BOP, CAL) [
11,
12]. Many studies conducted in the past confirm the effectiveness of the aPDT treatment using 5-delta aminolevulinic acid (5-ala) as the precursor of the photosensitizer PpIX in the heme biosynthesis [
12‐
15] which have encouraged the production of a sol–gel commercialized as ALADENT (ALAD), specially formulated to deliver this active compound 5-ala at 5% [
16‐
18]. A very appreciable aspect of this gel is its temperature-dependent state transition formulation based on a mixture of poloxamers that ensures the stability of the active ingredient [
19,
20]. Indeed, this 5% 5-ala thermolabile formulation is liquid and it is converted to gel with a temperature higher than 28 °C. The subsequent conversion into a gel leads to a controlled spreading of the active component and facilitates its topical administration. The presence of poloxamers improves the muco-adhesion properties of ALAD, making it suitable for skin treatments [
21]. Recent in vitro and in vivo studies have shown the capability of the ALAD-PDT protocol to inactivate pathogens involved in periodontal disease [
16,
17]. The 5-ala acts as a pro-drug being a precursor of the endogenous photosensitizer PpIX that, unlike other substances such as methylene blue and toluidine blue, has also proved effectiveness against oral bacterial and fungal biofilm [
16‐
18]. These studies demonstrated that the drug-light interval time (45 min + 7 min) is enough to treat oral infections by killing microbes, and provides the bacterial load reduction, together with the reduction of periodontal soft tissue inflammatory parameters, such as PD, CAL, and BOP [
4,
16,
22,
23]. Despite the proven strong antibacterial and antifungal effect of ALAD-PDT, which makes it an ideal product for treating periodontitis and peri-implantitis, there is no information regarding of ALAD-PDT potential effects on human cells. Literature poorly provides evidence on the effects of the PDT on healthy cells. Egli et al. revealed cytotoxicity of PDT in different human cell lines, and Bastian et al. obtained similar results in porcine osteoblasts and chondrocytes. Both authors applied the same PDT protocol also used as an anticancer strategy, in terms of time (4 h) and in terms of high light dose [
24,
25]. A different approach has been taken by Kushibiki et al. who use PDT doses lower than those conventionally used for cancer treatment and demonstrated the photochemical promotion of mouse and rat osteoblast cell differentiation via ROS production [
26]. However, these aforementioned protocols had incubation intervals too high considering that the objective of clinicians is to reduce the working time and consequently also the patient’s compliance. In ALAD-PDT, the incubation and time of irradiation were reduced to 45 min and to 7 min that are lower with respect to the PDT protocols investigated in the cited articles [
4,
16,
22,
23]. Based on these previous studies, the ALAD-PDT, until now tested just as antibacterial and antifungal strategy consisting of the thermo-gel containing 5% of 5-ala irradiated by the red LED light at 630 nm, was here investigated to verify if it may promote osteogenic effects. Hence, the purpose of this study was to explore the effects of ALAD gel at different concentrations applied for 45 min, in combination with 7 min of 630 nm red LED light (ALAD-PDT) on human oral osteoblasts.